1. Field of the Invention
The present invention relates to a photoelectric conversion device including a photodiode and an integrated circuit.
2. Description of the Related Art
Light-receiving elements used as optical sensors are roughly divided into three categories: a light-receiving element using photovoltaic effect, a light-receiving element using photoconductive effect, and a light-receiving element using photoemission effect. Among these light-receiving elements, the light-receiving element using photovoltaic effect, such as a photodiode and a phototransistor, has high response speed and low power consumption and can be easily miniaturized. The light-receiving element using photovoltaic effect can be applied to a wide range of applications such as optical communication, optical fiber, remote controllers, light pens, fax machines, illuminometers, color sensors, linear image sensors, spectrophotometers, and exposure meters.
A phototransistor generates current when a junction of semiconductors is irradiated with light. A photodiode generates current when a junction of semiconductors is irradiated with light. A phototransistor is more sensitive to light than a photodiode. On the other hand, although a photodiode has higher response time than a phototransistor, current obtained when the photodiode receives light is very small. Therefore, the photodiode is generally used in combination with an amplifier circuit which amplifies the current. An element in which a transistor included in an integrated circuit such as an amplifier circuit, a constant voltage circuit, or a Schmitt trigger circuit and a photodiode are formed on one chip is referred to as a photo IC (a photoelectric conversion device).
Patent Document 1 (Japanese Published Patent Application No. H6-029567) discloses a light-receiving circuit in which a photodiode and a thin film transistor are formed on one substrate.